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Detectives investigating a brutal 1995 cold murder case hope a record $100,000 reward will help flush out a guilty killer carrying around a "cognitive load of burden".

Angela Blackmoore, a 21-year-old mum who was 10 weeks' pregnant, was stabbed 39 times in her house on Vancouver Crescent in the Christchurch suburb of Wainoni on the blustery, warm Thursday evening of August 17, 1995.

Her 2-year-old son Dillon was sleeping in his bedroom at the time of her death.

Angela Blackmoore.

Her partner, Laurie Anderson, discovered her body when he returned home after work at 11.20pm that night.

Police never found her killer, despite interviews with hundreds of people, and the case has remained open for more than two decades.

Investigations manager Detective Inspector Corrie Parnell said it was a "horrific" crime which took the life of a young mother "with her whole life ahead of her".

"Since Angela's murder our focus has been on bringing the person responsible to justice, and although many years have passed that focus has not changed," Parnell said.

"With the passing of time allegiances may have changed and we're appealing to anyone with information that might help, no matter how small or seemingly insignificant, to contact us," he said.

Anyone with any information about the case could call a dedicated police phone number, 0800 22 1995.

Since the appeal was launched this morning, police have already received four phone calls from the public.

Any possible new leads will be explored, Parnell said.

The $100,000 reward is the largest ever offered by police. Parnell called it "unprecedented".

The cash would only be paid out if the tip-off led to the murderer being, firstly identified, and then convicted in a court of law.

The lead investigator remains Detective Sergeant Todd Hamilton, who was called out on the night of the murder and who has held the police file ever since.

The file has been reviewed several times over the past two decades.

More than 100 people have been spoken to during the course of the investigations.

However, there's never been enough evidence to lay charges.

Today there are less than 10 persons of interest remaining that haven't been ruled out, Parnell said.

History and experience, coupled with the brutality of the crime, suggests it may have been perpetrated by someone who knew her. There were no signs of forced entry.

However, he stressed the investigators have to keep an open mind in case someone comes from "left field".

While 24 years have passed, Parnell believes the killer's "cognitive load of burden" will not have passed.

"I encourage them to come forward and, secondly, anybody that may have been of assistance in the crime, or has material information or was close to those persons involved, that same burden will be present, and I encourage them to come forward," Parnell said.

Given the bloody scene of the murder aftermath, the killer or killers would've had to have undergone a clean-up operation, Parnell said.

Fingerprints were taken from the house and remain unidentified. They have been kept on the police national DNA database, along with other items taken from the scene, and are regularly checked as advances in DNA testing keep getting better.

Parnell said he lives in hope of one day getting a call from forensic experts who have nailed a trace match.

Blackmoore's partner Laurie Anderson has been kept informed by investigators.

However, her parents have since passed away.

"What drives the investigation team is to get this resolved," Parnell said.